1. Field of the Invention
This invention relates to a process for producing a mesophase pitch for high quality carbon products such as carbon fibers, needle-like pitch cokes, etc., and carbon products, particularly carbon fibers, produced from the mesophase pitch.
2. Description of the Prior Art
Carbon products such as carbon fibers, needle-like cokes or synthetic graphite electrodes made of the mesophase pitch as raw material have a graphite-like crystal structure, and thus have distinguished mechanical and electrical characteristics such as high Young's modulus, high electroconductivity, etc.
According to the conventional process for producing carbon products from pitches, the softening point, molecular weight, etc. of a pitch are adjusted by heat treatment, extraction, etc. to give a raw material for carbon products. Thus prepared pitch is then molded into desired shapes, for example, fiber form, etc., and carbonized or graphitized.
Generally, optically anisotropic portions called "mesophase" are formed in an optically isotropic pitch, as the pitch is heated, and the mesophase portions gradually increase in their proportion through repetition of their growth, agglomeration, and deformation.
The mesophase has a liquid crystal structure in which planar condensed aromatic molecules are regularly oriented. Mesophase pitch having a high degree of orientation can be readily converted to graphite-like crystals by carbonization and graphitization, and thus carbon products having a well developed graphite-like structure can be obtained from such a mesophase pitch.
On the other hand, the production of carbon products from a pitch requires a molding step, and thus the pitch must have a good moldability. To this end, the mesophase pitch must have a good flowability.
For example, in the production of carbon fibers from a pitch, the degree of crystal size and the degree of crystal orientation in carbon fibers greatly depend on whether the condensed aromatic molecules in the pitch for carbon fibers can be oriented in the axial direction of fibers in the melt-spinning step or not. Thus, it can be said that the desirable pitch for the carbon fibers must be a mesophase pitch containing a group of regularly oriented condensed aromatic molecules and also must have a sufficient flowability. These are common requirements for all the raw material pitches for synthetic graphite products.
Usually the condensed aromatic molecules grow larger and the content of mesophase becomes higher. The regularity and orientation are improved, but at the same time the softening point will become higher, resulting in a lower flowability and poorer workability. In the production of carbon fibers, the pitch having a substantially 100% mesophase content can hardly flow when subjected to melt-spinning. When the spinning temperature is elevated to obtain a sufficient flowability, the pitch will be partially decomposed or sometime coked.
Thus, it has been so far desired in the production of a raw material pitch for carbon products to produce a mesophase pitch having a lower softening point, in other words, a pitch having a higher mesophase content, so long as the softening point is on the same level. In the case of a mesophase pitch having a good flowability, mesophase spherulites themselves can readily agglomerate mutually during heat treatment to give large domains or continuous mesophases can be developed. Solubility in a solvent such as quinoline, etc. is one of indices for evaluating the characteristics of the mesophase pitch. A mesophase pitch containing a quinoline-soluble mesophase has a lower melting point and a higher flowability, and thus has an advantage of easy melt-spinning for the production of carbon fibers. Thus, a mesophase pitch has been now regarded as the most desirable raw material for producing high performance carbon fibers, and studies of the processes have been so far extensively made. Some of the so far proposed processes for producing a mesophase pitch containing a quinoline-soluble mesophase are given below:
U.S. Pat. No. 4,209,500 discloses production of a mesophase pitch having a substantially 100% mesophase content and containing a pyridine-soluble mesophase by passing an inert gas through a pitch heated and stirred at 380.degree. to 430.degree. C., where a treatment time of 2 to 60 hours and a large amount of the inert gas are required.
U.S. Pat. No. 4,208,267 discloses production of pitch portions, which can be readily converted to a mesophase pitch containing a quinoline-soluble mesophase by treating a pitch with a specific solvent, where the pitch portions are called neomesphase-forming fractions (NMF fractions), but the NMF fractions obtainable from the pitch are very small.
U.S. Pat. No. 4,184,942 discloses an increased production of NMF fractions by heat-treating a pitch in advance, followed by separation of NMF fractions, where the heat treatment, solvent extraction, and further heat treatment must be carried out, resulting in complicating of the process.
As described above, the prior art processes require a large amount of a special gas, or a specific solvent, or a complicated process or prolonged treatment time for producing a mesophase pitch containing a quinoline-soluble mesophase, and thus still have problems to be solved.